1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor. In addition, the present invention relates to an image forming method and apparatus using a photoreceptor.
2. Discussion of the Background
The following photosensitive layers have been used for electrophotography photoreceptors:
(1) layers mainly constituted on selenium or a selenium alloy;
(2) layers in which an inorganic photocoductive material such as zinc oxide or cadmium sulfide is dispersed in a binder resin;
(3) layers using an organic photoconductive material such as a combination of poly-N-vinylcarbazole and trinitrofluorenone, and azo pigments; and
(4) layers using amorphous silicon.
The electrophotographic image forming methods typically include the following processes:
(1) charging an electrophotographic photoreceptor in a dark place (charging process);
(2) irradiating the charged photoreceptor with imagewise light to form an electrostatic latent image thereon (light irradiating process);
(3) developing the latent image with a developer including a toner mainly constituted of a colorant and a binder to form a toner image thereon (developing process);
(4) optionally transferring the toner image on an intermediate transfer medium (first transfer process);
(5) transferring the toner image onto a receiving material such as a receiving paper ((second) transfer process);
(6) heating the toner image to fix the toner image on the receiving material (fixing process); and
(7) cleaning the surface of the photoreceptor (cleaning process).
In such image forming methods, requisites for the photoreceptors are as follows:
(1) to be able to be charged so as to have a proper potential in a dark place;
(2) having a high charge retainability (i.e., the charge formed thereon hardly decays in a dark place); and
(3) to rapidly decay the charge thereon upon application of light thereto.
Currently, the photoreceptors using organic photosensitive materials are widely used because of satisfying such requisites as mentioned above and having the following advantages over the other photoreceptors:
(1) manufacturing costs are relatively low;
(2) having good designing flexibility (i.e., it is easy to design a photoreceptor having a desired property); and
(3) hardly causing environmental pollution.
As for the organic photoreceptors, the following photosensitive layers are known:
(1) a photosensitive layer including a photoconductive resin such as polyvinyl carbaozole (PVK) or the like material;
(2) a charge transfer photosensitive layer including a charge transfer complex such as a combination of polyvinyl carbaozole (PVK) and 2,4,7-trinitrofluorenone (TNF) or the like material;
(3) a photosensitive layer in which a pigment, such as phthalocyanine or the like, is dispersed in a binder resin; and
(4) a functionally-separated photosensitive layer including a charge generation material and a charge transport material.
Among these organic photoreceptors, the photoreceptors having a functionally-separated photosensitive layer especially attract attention now.
The mechanism of forming an electrostatic latent image in the functionally-separated photosensitive layer having a charge generation layer and a charge transport layer formed on the charge generation layer is as follows:
(1) when the photosensitive layer is exposed to light after being charged, light passes through the transparent charge transport layer and then reaches the charge generation layer;
(2) the charge generation material included in the charge generation layer absorbs the light and generates a charge carrier such as electrons and positive holes;
(3) the charge carrier is injected to the charge transport layer and transported through the charge transport layer due to the electric field formed by the charge on the photosensitive layer;
(4) the charge carrier finally reaches the surface of the photosensitive layer and neutralizes the charge thereon, resulting in formation of an electrostatic latent image.
For such functionally-separated photoreceptors, a combination of a charge transport material mainly absorbing ultraviolet light and a charge generation material mainly absorbing visible light is effective and is typically used. Thus, functionally-separated photoreceptors satisfying the requisites as mentioned above can be prepared.
Currently, needs such as high speed recording, high durability and upsizing are growing for electrophotographic image forming apparatus. Therefore, an increasing need exists for photoreceptors having high reliability, which can produce good images even when repeatedly used for a long period of time while having the above-mentioned requisites.
In general, photoreceptors have a drawback such that when the photoreceptors are repeatedly used in image forming apparatus, the potentials of the dark and lighted areas serious vary. One reason for such variation in the electrostatic properties is abrasion of the photosensitive layer. Currently, photoreceptors are used for a long period of time for the reasons mentioned above, and therefore the surface of the photoreceptors tends to be abraded, resulting in deterioration of the above-mentioned electrostatic properties. Therefore, photoreceptors having good mechanical durability have been investigated. On the other hand, various measures to reduce the abrasion of a photoreceptor have been investigated at the image forming apparatus side.
Another reason for the variation in the electrostatic properties of photoreceptors is that the materials used in the photosensitive layer are chemically deteriorated by the substances generated in image forming apparatus, such as ozone and nitrogen oxides (NOx). In particular, it is a serious problem that when the surface potential of a photoreceptor is decreased due to such substances, the image qualities of the resultant images deteriorate.
In attempting to prevent such decrease of surface potential of photoreceptors, methods in which an additive having an anti-oxidation function is included in charge transport layer thereof have been disclosed in Japanese Patent Publications Nos. 50-33857 and 51-34736, and Japanese Laid-Open Patent Publications Nos. 56-130759, 57-122444, 62-105151 and 3-278061.
However, it is found by the present inventors"" investigation that such methods have a drawback such that the potential in a lighted area increases.
On the other hand, halogen-containing solvents such as monochlorobenzene, dichloromethane and the like, which are typically used for coating charge transport layers, are considered to adversely affect the natural environment and human being. In order to protect environment, it is needed that halogen-containing solvents are not used when charge transport layers are formed.
Halogen-containing solvents are used in charge transport layer coating liquids to dissolve a polycarbonate resin which is typically used as a binder resin in charge transport layers. Tetrahydrofuran, dioxane, xylene, toluene, methyl ethyl ketone, cyclohexanone etc. can be used as a substitute for halogen-containing solvents. Among these substitutes, tetrahydrofuran is the most preferable in view of preservability, productivity, and coating properties of coating liquids such as evenness of thickness of the resultant charge transport layer.
However, tetrahydrofuran is easily oxidized and thereby explosive peroxides are generated. Therefore, a small amount of a phenolic antioxidant is typically included in tetrahydrofuran to prevent the oxidation reaction,.
It is found by the present inventors that when tetrahydrofuran including such a phenolic antioxidant is used to form a charge transport layer and in addition a protective layer including a filler is formed thereon, a problem occurs such that the potential in the lighted area of the resultant photoreceptor increases when the photoreceptor is repeatedly used.
Because of these reasons, a need exists for a photoreceptor which can produce images having good image qualities while having a long life and high reliability and which is friendly to the environment.
Accordingly, an object of the present invention is to provide a photoreceptor which can produce images having good image qualities while having a long life and high reliability and which is friendly to the environment.
Another object of the present invention is to provide an image forming method and apparatus in which images having good image qualities are stably produced for a long period of time.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by an electrophotographic photoreceptor including an electroconductive substrate, a charge generation layer overlying the substrate, a charge transport layer formed overlying the substrate and including a charge transport material, a polycarbonate resin, an antioxidant having the following formula (1): 
and a sulfur-containing compound having an alkyl group having from 6 to 30 carbon atoms, and a protective layer formed as a top layer and including a binder resin and a filler dispersed in the binder resin. The sulfur-containing compound is one having an alkyl group containing 6 to 30 carbon atoms. Preferably the sulfur-containing compound has the following formula (2): 
wherein n is an integer of from 6 to 30.
The photoreceptor may further include an undercoat layer on the substrate. Preferably the charge generation layer is formed on the undercoat layer and the charge transport layer is formed on the charge generation layer.
The total thickness D of the charge transport layer and the protective layer is preferably from 10 xcexcm to 30 xcexcm, and more preferably from 10 xcexcm to 26 xcexcm.
The charge transport layer is preferably formed by coating a coating liquid including tetrahydrofuran, the charge transport material, the polycarbonate resin, the antioxidant having formula (1) and the sulfur-containing compound. In addition, the charge transport layer may further include tetrahydrofuran.
In another aspect of the present invention, an image forming method is provided which includes the steps of charging a photoreceptor such that the photoreceptor has a surface potential V; imagewise irradiating the photoreceptor with a light beam to form an electrostatic latent image thereon, wherein the photoreceptor is the photoreceptor mentioned above, and wherein the image forming method satisfies the following relationship:
12xe2x89xa6V/Dxe2x89xa640 
wherein V represents the surface potential of the photoreceptor in a unit of volt; and D represents the total thickness of the charge transport layer and protective layer in a unit of micrometer.
The diameter of the light beam is preferably not greater than 60 xcexcm.
In yet another aspect of the present invention, an image forming apparatus is provided which includes at least a photoreceptor, a charger, a light irradiator, and an image developer, wherein the photoreceptor is the photoreceptor of the present invention and wherein the charger is a contact charger in which the charging element charges the photoreceptor while contacting the photoreceptor or a short-range charger in which the charging element charges the photoreceptor while the charging element is arranged closely to the photoreceptor.
The charging element preferably applies a DC voltage overlapped with an AC voltage to the photoreceptor when charging the photoreceptor.
In a further aspect of the present invention, a process cartridge is provided which includes at least a housing and a photoreceptor, wherein the photoreceptor is the photoreceptor of the present invention.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.